1. Field of the Invention
This invention relates in general to a recording head for recording or printing images such as characters and graphical representations, by applying an electric current to a recording medium or a ribbon or film or other form of intermediate member interposed between the recording medium and the recording head. More particularly, the present invention is concerned with the construction of a distal end portion of such a recording head at which the head contacts the recording medium or intermediate member.
2. Discussion of the Prior Art
Various types of recording heads for recording by application of an electric current to a recording medium or an intermediate member are known. In particular, there is known a recording head having a laminar or multi-layered structure which includes a substrate or substrates, and an array of recording electrodes and an array of return circuit electrodes which are supported by or formed on the substrate or substrates. Examples of this type of recording head are disclosed in laid-open Publication Nos. 61-35972, 62-292461, 54-141140, 58-12790 and 61-230966 of unexamined Japanese Patent Applications.
As disclosed in the publications identified above, the recording head of the type indicated above is adapted such that an electric current is applied to an electrically resistive or conductive layer formed or coated on or carried by a suitable recording medium or a suitable planar intermediate support member in the form of a sheet, film or ribbon. The electrically resistive or conductive layer may be formed on a roller or other support member, or constitute an inner layer of the recording medium or support member. In a recording operation using an intermediate ribbon or film having an electrically resistive layer and an ink layer, for example, an electric current is applied to the resistive layer through the recording head and causes Joule heat to be generated by the resistive layer. Selected local areas of the ink layer are then heated, and the ink material in these heated local areas is fused, vaporized or diffused. As a result, the ink material is transferred to the appropriate local areas of the recording medium so as to form a black or colored image. If a electric current is applied directly to a recording medium, the appropriate local areas of the medium are suitably colored due to Joule heat generated by an electric current, or due to removal of the covering material from the medium surface due to an electrical discharge occurring thereon.
The electrically resistive layer provided on the recording medium or intermediate support member may be an electrically conductive layer, an electrically conductive or resistive ink layer (which serves also as an ink-bearing layer), a heat-sensitive layer having an electrolyte, or any form of layer through which an electric current may flow.
In a recording or printing operation by the recording head for use with the recording medium or intermediate support member as described above, the recording electrodes and the return circuit electrode or electrodes must be held in electric contact with the electrically resistive layer of the recording medium or support member. To this end, the electrodes used in the known recording heads as disclosed in the publications indicated above are formed of a material which has a higher degree of wear resistance than the material of the substrate structure and the material of an electrically insulating layer used for the heads.
However, the mere selection of the materials suitable for the electrodes, substrate structure and insulating layer is not sufficient for maintaining good electrical contact of the electrodes with the electrically resistive layer for a prolonged period of time. As the accumulative operating time of the recording head increases, one of the recording electrode array and the return circuit electrode array is worn to a greater extent than the other electrode array, causing poor electrical contact of that electrode array with the electrically resistive layer, or separation of the electrode array from the substrate due to friction therebetween. Thus, it is difficult to maintain good electrical contact between the electrodes and the resistive layer, for a sufficiently long period.
For the recording head to maintain excellent operating characteristics with minimum crosstalk and to improve the print quality, it is required that the distance between the recording electrodes and the return circuit electrodes be kept small and constant. If an electrically insulating layer interposed between the two electrode arrays has a relatively small thickness to satisfy the above requirement, it is extremely difficult to form such an insulating layer or control its thickness, when the recording head is fabricated. Further, the insulating layer having a small thickness may result in reducing the mechanical strength of the recording head. For this reason, the above approach to assure excellent operating characteristics of the recording head is not practically available.
Another approach that can be considered to assure excellent operating characteristics of the recording head is to use two substrates which support respective arrays of electrodes, i.e., recording electrodes and return circuit electrodes. These two substrates are bonded together by an adhesive with an electrically insulating layer interposed between the major surfaces of the two substrates on which are formed the electrode arrays. This laminar structure, however, must include two adhesive layers, one between the insulating layer and the first substrate, and the other between the insulating layer and the second substrate. The adhesive layers tend to have different thickness values, depending upon the amount of an adhesive agent applied or the specific manner of application of the adhesive agent. Further, the thickness of each adhesive layer tends to vary from one portion of the layer to another. Accordingly, it is difficult to accurately control the distance between the recording and return circuit electrodes, and is therefore difficult to improve the printing quality attained by the recording head.